The present invention relates to an oil-based ink for an ink jet process of an electrostatic type which is used for the formation of image by an ink jet recording system of an electrostatic type (electrostatically inducing type or electrostatically attractive type).
The ink jet recording system is a recording method capable of printing at a high speed in low noise, and recently has become rapidly widespread.
The ink jet recording system comprises discharging a liquid ink having high fluidity from a narrow nozzle to recording paper and is divided into a continuous discharging system and an on-demand discharging system. The continuous discharging system includes an electrostatic type (Sweet type, Hertz type), and the on-demand discharging system includes a piezoelectric type, a thermal ink jet type and an electrostatically accelerating type, as well known in the art.
Of the ink jet recording system of on-demand type utilizing static electricity, a method called an electrostatically accelerating type ink jet or slit jet is known as described in Susumu Ichinose and Yuuji Ooba, Denshi Tsusin Gakkai Ronbunshi, Vol. J66-C, No. 1, page 47 (1983) and Tadayoshi Oono and Mamoru Mizuguchi, Gazo Denshi Gakkaishi, Vol. 10, No. 3, page 157 (1981). Such an ink jet recording method is also described more specifically, for example, in JP-A-56-170, JP-A-56-4467 and JP-A-57-151374 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d).
According to the method, ink is supplied from an ink tank to a slit-shaped ink chamber having many electrodes arranged in inner surfaces of slit-shaped ink retaining part and when a high voltage is selectively applied to each electrode, the ink neighboring to the electrode is discharged on a recording paper closely positioned against the slits, thereby conducting recording.
A method which does not use such a slit-shaped recording head is also known. In JP-A-61-211048, there is described a method in which pores of a film-like ink retainer having plural pores are filled with ink and the ink in the pores is transferred to a recording paper by applying selectively a voltage to the ink using a multi-needle electrode.
It is believed that the theory of the ink flight is that the high voltage applied to the electrode arranged pours electric charges into the ink adjacent to the electrode and the charged ink neighboring to the electrode is ejected upon the electrostatic power generated. Therefore, the ink is normally not charged but only when the voltage is applied, the ink neighboring to the electrode is charged to get power for the ejection.
The ink employed for these methods is that having electric resistance of from about 106 to about 108 xcexa9cm. Since water has low electric resistance, an oily solvent containing a coloring agent such as a dye dispersed therein with a dispersing aide such as a surface active agent to control the electric resistance thereof is ordinarily employed.
Further, various proposals for controlling the properties of oil-based ink have been made. For example, there are a method of controlling viscosity and specific resistance of the oil-based ink as described in JP-B-52-13127, a method of controlling dielectric constant of a dispersion medium used in the ink and specific resistance of the ink as described in JP-A-53-29808, and a method of varying the dispersion medium for the oil-based ink or a method of incorporating a specific compound into the ink composition as described in JP-A-3-79677, JP-A-3-64377, JP-A-4-202386 and JP-A-7-109431.
However, the oil-based ink using these known techniques are still insufficient in view of their properties, for example, preservation stability of the oil-based ink, reproducibility of recording image at the time of repeated use of the ink, blur of the ink on an ink receiving material, clogging of the ink in a nozzle or an ink delivery channel, and stability on ink ejection. More improvements in these properties have been therefore desired.
Another ink jet technique of an electrostatic type is described in WO 93/11866. This method comprises, after supplying ink containing charged particles or particles which are chargeable under an electric field dispersed therein to an ink ejection apparatus, a series of steps composed of (1) forming ink meniscus at the tip of an ejection electrode for ejecting the ink, (2) increasing particle density in the ink meniscus upon electrophoretic concentration of particles, and (3) ejecting agglomerations of the particles away from the ejection electrode by forming an electric field between the ejection electrode and a counter electrode bearing a recording medium.
This method which does not use a nozzle structure contrary to conventional methods has many advantages in that ink containing dispersed particles such as pigment can be ejected as minute droplets having a particle size of several xcexcm, in that the droplet ejected can have a high concentration of particles, and in that a dot size of image can be varied by changing a size of the droplet by means of controlling an ejection signal.
Accordingly, images composed of pigment having good light-fastness and water-resistance can be formed and clear images of high resolution and high density including continuous gradation dot images can be obtained.
The oil-based ink to be used include those comprising an electrically insulating liquid having an electric resistance of 109 xcexa9cm or more containing insoluble and chargeable particles and charging agents therein as described in WO 95/1404 and WO 96/10058.
Oil-based ink in which an amount of charge of particles or an average particle size of particles is defined in the specific range as described in JP-A-9-193389 and JP-A-8-291267, and oil-based ink in which thermal properties of solid material in the ink composition are specified as described in JP-A-9-137094 are also proposed.
However, when ink jet recording was conducted using such known oil-based ink, due to unstable ink ejection or insufficient concentration of pigment particles in the ink, disappearance or blur of image formed and poor image density, particularly in a solid image area were observed. Further, in case of using ink preserved for some time, an ejection condition such as a voltage applied was varied from the use of fresh ink and a rate of concentration and ejection remarkably changed. That is, a problem that quality of image obtained varied depending on the preservation condition of the ink arose.
With development in business appliances and progress in office automation in recent years, a plate-making system wherein an image is formed on a lithographic printing plate precursor comprising a water-resistant support having provided thereon an image receiving layer in a various manner to prepare an offset lithographic printing plate has become widespread in the field of small-scale commercial printing. The ink jet recording method can be employed for the image formation in such a plate-making system. However, when printing was actually conducted using a printing plate having clear images formed using conventional oil-based ink, only several hundred sheets of prints having clear images could be obtained. This is clearly not satisfactory to printing durability. Specifically, there is a problem that the strength of ink particles fixed on the printing plate is insufficient for offset printing.
Therefore, an object of the present invention is to provide an oil-based ink for an ink jet process of an electrostatic type which is excellent in ejection stability of ink, clear image formation and strength of image.
Another object of the present invention is to provide an oil-based ink for an ink jet process of an electrostatic type which prepares a printing plate capable of providing a large number of prints having clear images.
A further object of the present invention is to provide an oil-based ink for an ink jet process of an electrostatic type which is excellent in redispersibility and preservation stability of dispersed particles, does not cause clogging in an ink delivery channel and ensures stable ink ejection.
Other object of the present invention will become apparent from the following description.
It has been found that these objects of the present invention are accomplished by an oil-based ink for an ink jet process of an electrostatic type comprising chargeable resin particles dispersed in a nonaqueous carrier liquid having an electric resistance of 109 xcexa9cm or more and a dielectric constant of 3.5 or less, wherein the resin particles dispersed are copolymer resin particles obtained by polymerization granulation of a solution containing at least one monofunctional monomer (A) which is soluble in a nonaqueous solvent and becomes insoluble in the nonaqueous solvent by polymerization, at least one monofunctional monomer (B) having an amino group represented by formula (I) shown below and being copolymerizable with monomer (A), and at least one resin for dispersion stabilization (P) which is a polymer soluble in the nonaqueous solvent and comprising a component represented by formula (II) shown below, a part of the polymer main chain being crosslinked: 
wherein R1 and R2, which may be the same or different, each represents a hydrogen atom or a hydrocarbon group having from 1 to 22 carbon atoms, or R1 and R2 may combine with each other to form a ring together with the nitrogen atom; 
wherein V0 represents xe2x80x94COOxe2x80x94, xe2x80x94OCOxe2x80x94, xe2x80x94(CH2)rCOOxe2x80x94, xe2x80x94(CH2)rOCOxe2x80x94, xe2x80x94Oxe2x80x94 or 
(wherein X represents a direct bond, xe2x80x94Oxe2x80x94, xe2x80x94OCOxe2x80x94 or xe2x80x94COOxe2x80x94); r represents an integer of 1 to 12; L represents an alkyl group having from 8 to 32 carbon atoms or an alkenyl group having from 8 to 32 carbon atoms; a1 and a2, which may be the same or different, each represents a hydrogen atom, a halogen atom, a cyano group, a hydrocarbon group, xe2x80x94COOxe2x80x94D1 or xe2x80x94COOxe2x80x94D1 linked through a hydrocarbon group (wherein D1 represents a hydrogen atom or a hydrocarbon group).